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Full text of "Diseases Of The Nose Throat And Ear"

CHAPTER 62
TYMPANOPLASTY

The techniques of tympanoplasty are concerned with the restoration of
hearing by reconstruction of the sound-conducting mechanism damaged
in whole or part by middle ear disease or as the result of surgery essential
for the elimination of chronic infection. The introduction of antibiotics and
the operating microscope have contributed greatly to the development of
these operations.

The basic requirements for successful results are: (1) Control of infection.
(2) The presence of good cochlear function. (3) Patency of the Eustachian
tube. (4) Mobility of the oval and round window membranes.

The normal transmission of sound waves to the fluid of the inner ear is
dependent not only upon mobility of the membranes of the oval and round
windows, but also upon a greater proportion of sound energy being trans-
mitted via the tympanic membrane, the chain of ossicles and the oval window
(the sound-pressure transformer mechanism), than that which reaches the
round window across the air in the tympanum. The baffle mechanism (sound
protection) is provided by the intact tympanic membrane which ensures
that sound waves reach the two windows in different phases. In addition,
the sound pressure effect is increased by the hydraulic and lever actions
of the tympanic membrane and ossicular chain on the oval window. The
changes in the drumhead, ossicular chain and window regions which result
from disease may cause not only a reduction in the total amount of sound
energy transmitted to the oval window, but also a reduction in the difference
of the amount of sound reaching the two windows. Theoretically, if sound
reaches the two windows with equal energy and at the same time, perilymph
being non-compressible, neither window membrane will move and no
disturbance of perilymph or basilar membrane will occur. In practice, how-
ever, a slight difference in impedance and a certain amount of bone conduc-
tion provide retention of some hearing. The objective of tympanoplasty is
therefore the preservation or recreation of the disproportionate conductivity
of the two fenestrae.

Techniques in tympanoplasty have developed rapidly in the past 25 years.
There has been a wide diversity of tissues used for reconstruction in myringo-
plasty and ossiculoplasty. Autograft temporalis fascia is widely used in the
repair of perforations while for simpler types of ossicular discontinuity the
patient's own incus can be remodelled and repositioned. The use of homo-
graft materials has increased steadily and is preferable to the use of inert
substances.

Considering the principles already described, the procedures used in
reconstruction vary according to the deficiency of the sound transmission
mechanism of which five types have been described: